Atomic beam tube having an improved coaxial cavity



Jan. 14, 1969 J. H. HOLLOWAY 3,422,456

ATOMIC BEAM TUBE HAVING AN IMPROVED COAXIAL CAVITY I Filed Aug. 18, 1964 Sheet .4 of 2 PRIOR ART in. 6 ix (@J, v a

PRIOR ART INVENTOR. JOSEPH H. HOLLOWAY ATTORNEY PRIOR ART FEGJ Jan. 14, 1969 J. H. HOLLOWAY 3,422,456

ATOMIC BEAM TUBE HAVING AN IMPROVED COAXIAL CAVITY Z of 2 Sheet Filed Aug. 18. 1964 INVENTOR; JOSEPH H.HOLLOWAY ATTORN EY United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE An atomic beam tube with a circular electric mode C-field cavity resonator. The cavity comprises a pair of coaxial tubular members closed at their ends and centrally excited by a waveguide or coaxial line. The cavity may be easily fabricated of low thermal expansion material such as quartz.

The present invention relates in general to atomic beam tubes and more particularly to an improved atomic beam tube utilizing a novel circular electric mode C-field resonator whereby improved frequency stability is obtained and ease of fabrication facilitated. Such improved atomic beam tubes are useful, for example, as atomic clocks and as frequency standards.

Heretofore, it has been proposed to use a Y-shaped C- field resonator comprised of a pair of axially Spaced circular electric mode resonant chambers coaxially aligned with the atomic beam path and excited by and interconnected by a resonant section of rectangular wave- 'guide center fed by a transmission line; see U.S. application No. 385,202 filed July 27, 1967, inventor, Robert F. C. Vessot, now US. Patent No. 3,348,040 and assigned to the same assignee as the present invention. This prior type of C-field cavity construction offers many electrical advantages primarily because it eliminates phase shift of the applied cavity magnetic fields in the pair of spaced beam field interaction regions taken in a direction transverse to the beam thereby greatly increasing the frequency stability of the atomic resonance of the beam particles.

However, problems of construction are introduced by this prior cavity geometry since the abrupt waveguide transitions from rectangular waveguide to the end cylindrical resonant chambers breaks the composite cavity into several coupled resonant systems. These chambers and transitions must be constructed and adjusted with great care to prevent setting up undesired phase shifts between the fields of the circular electric modes in the pair of spaced apart beam field interaction regions. In addition, the proposed composite rectangular and circular cavity geometry is ditficult to construct of materials having low temperature coefficients of expansion such as quartz because of complex geometry of the composite cavity.

In the present invention, a circular electric mode C- field cavity having greatly simplified geometry is provided by forming the pair of circular electric mode resonators in the end sections of a coaxial mode resonant chamber disposed ooaxially of the end chambers and therebetween. This improved cavity is simple in geometry since it comprises only a pair of coaxial tubular members closed at their ends and centrally excited by waveguide or coaxial line. With the simplified cavity geometry the C-field cavity may be easily fabricated of low thermal expansive material such as quartz which is suitably plated with conducting material such as silver. In addition, the transitions in the composite cavity from the coaxial resonant chamber to the end cylindrical resonant chambers are less pronounced and therefore the undesired phase shifts introduced by such transitions are more easily controlled or eliminated.

"'ice The principal object of the present invention is the provision of an improved atomic beam tube.

One feature of the present invention is the provision of an improved circular electric mode C-field resonator structure wherein a pair of circular electric mode resonator portions are interconnected by a section of coaxial transmission line whereby the geometry of the composite resonator structure is simplified.

Another feature is the same as the preceding feature wherein the coaxial transmission line section is coaxially aligned with the circular electric mode sections and with the beam path.

Another feature is the same as the preceding feature wherein the circular electric mode resonator is formed by a pair of coaxially disposed axially co-extensive quartz tubes plated with a conductive material and closed at their ends to form a circular electric mode resonator whereby the cavity resonator is rendered non-responsive to temperature fluctuations.

Another feature is the same as any of the preceding features wherein the walls that close-off the ends of the circular electric mode resonator are formed of metal and include adjustable centrally disposed segments for trimming the tuning of the composite resonator structure and which conductive end walls may be made of the material having a higher coefiicient of thermal expansion than the quartz side walls for temperature compensation.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a longitudinal sectional view of an atomic beam tube employing the features of the prior art;

FIG. 2 is an enlarged longitudinal sectional view of a preferred C-field cavity embodying features of the present invention;

FIG. 3 is a fragmentary perspective view of a portion of the structure of FIG. 2 delineated by line 3-3;

FIG. 4 is a transverse sectional view of a portion of the structure of FIG. 1 taken along line 4-4 in the direction of the arrows, and

FIG. 5 is a transverse sectional view of a portion of the structure of FIG. 1 taken along line 55 in the direction of the arrows.

Referring now to FIG. 1 there is shown a prior art atomic resonance beam tube apparatus. More specifically, the atomic beam tube includes an elongated combined tubular envelope and hollow cylindrical shaped support structure 2 as of non-magnetic stainless steel. The tube construction will be more fully described below. Contained within the envelope 2 is a source 3 of atomic beam particles such as, for example, cesium or thallium atoms which projects the beam particles axially of the tube structure over an elongated beam path 4. A beam particle detector 5 such as a conventional hot wire ionizer is disposed at the terminal end of the beam path for detecting resonance of the beam.

A circular electric C-field cavity resonator 6 forming the subject matter of the aforementioned application Ser. No. 385,202, now US. Patent No. 3,348,040 is disposed midway between the beam source 3 and detector 5 for exciting atomic resonance of the beam particles by an alternating magnetic field component H in the presence of a DC. polarizing magnetic C-field component H When the atomic beam tube is being used as a frequency standard or atomic clock, the beam particles are preferably resonated in a field independent transition or resonance and for this condition the alternating R.F. magnetic field H at the atomic resonance frequency should have a strong component parallel to a DC. polarizing magnetic field component H commonly called the C-field.

A particularly advantageous combination of C-field magnet and C-field cavity resonator structure 6 is obtained when the circular electric mode resonator 6 is used with a cylindrical magnetically shielded C-field solenoid 7 for producing the axially directed polarizing C-field' H along the beam path within the resonator 6. The cylindrical solenoid 7 yields a very uniform polarizing magnetic C- field and is described and claimed in my copending US. patent application Ser. No. 366,493 titled, Atomic Resonance Method and Apparatus With Improved Magnetic Field Homogeneity Control, filed May 11, 1964, and issued as U.S. Patent No. 3,345,581, inventor Robert F. C. Vessot, and assigned to the same assignee as the present invention.

The circular electric mode resonator structure 6 comprises a pair of axially spaced-apart cylindrical resonator chambers 8 coaxially aligned with the beam path 4. Each cylindrical resonator 8 is provided with a pair of apertures 9 in the end walls in registry with the beam path 4 to permit passage of the beam therethrough. An axially directed section of rectangular waveguide 11 interconnects the two cylindircal chambers 8 and is coupled to each chamber 8 via the intermediary of irises 12.

Wave energy is fed into the rectangular waveguide 11, at a point preferably midway of its length, via the intermediary of a suitable coupling device, such as a conventional magnetic coupling loop 13, which is excited by a coaxial line 14. The rectangular section of guide 11 preferably has high Q as taught in my co-pending application Ser. No. 340,767, now US Patent No. 3,354,307. In addition, the coaxial line includes a high loss section 15 and a reflective discontinuity 16 disposed between the high loss section and a source of microwave energy, not shown, at the atomic resonance frequency connected at terminal 17 of the coaxial line 14.

The C-field resonator 6 structure is defined by the composite-coupled coaxial line sections 15, 16, waveguide 11, and cylindrical chambers 8. The cylindrical chambers 8 are dimensioned to support a dominant circular electric TE mode at the atomic resonance frequency when excited with wave energy coupled through irises 12. The resonant section of transmission line or waveguide 11 and coupled chambers 8 define a high Q portion of the composite resonator structure 6. The low Q portion includes the high loss section 15 of coaxial line 14 intermediate the coupling loop 13 and the reflective discontinuity to thereby provide a low Q composite resonator to prevent thermal detuning effects while at the same time providing a high Q portion to prevent undesired phase shifts between the R.F. magnetic fields in the spaced chambers 8 due to energy absorption in the high Q portion.

The mounting of the shielded solenoid 7 and cavity structure 6 is more fully described below with regard to a preferred embodiment of the present invention shown in FIG. 2.

A pair of magnetically shielded state selecting magnets 18 and 19 are disposed on opopsite ends of the resonator structure 6, respectively. In a preferred embodiment, magnets 18 and 19 are quadrupole or hexapole magnets to obtain a focusing of the beam as well as state selection. Magnet 18 is disposed between the source 3 and the resonator 6. Magnet 18 focuses out of the beam atomic particles of one energy state and focuses in toward the center of the beam particles of the other energy state. Either state may be selected for the beam by merely intercepting the unwanted beam particles. However, by selecting the atomic energy state which is focused in toward the center of the beam an additional advantage is obtained due to the increased beam density yielding smaller beam crosssectional areas for a given beam fiux intensity. Smaller beam cross-sectional area leads to smaller transverse phase shifts in the applied R.F. resonating magnetic field H in the beam-field interaction regions and thereby yields greater frequency stability of the tube.

The spaced resonator chambers 8 provide the pair of axially spaced beam-field interaction regions for resonat ing the atomic beam particles. The resonant fields H in the spaced chambers 8 may be selected for in phase operation or out of phase operation, depending upon whether peak or null detection is desired.

In the apparatus shown, for in phase operation of chambers 8, at atomic resonance of the beam, the particles will be deflected out of the beam by the second state selecting magnet 19. Thus, resonance of the beam will appear as a minimum in the detected beam current of detector 5 when using a button detector 5 and a path amplitude signal will appear when using a ring or annular detector. If the phase of the RF. fields in chambers 8 is 180 out of phase (out of phase operation) then, with the button detector apparatus shown, at resonance there would be a peak in amplitude of the detected beam current at button detector 5 and a minimum if an annular detector were utilized instead of the button detector.

Referring now to FIG. 2, there is shown a preferred circular electric mode cavity resonator structure 6 incorporating features of the present invention which is described but not claimed specifically in the aforementioned prior co-pending application. The circular electric mode cavity in this instance is defined by an outer quartz cylinder or tube 22 coaxially disposed of the beam path 4. The cylinder 22 is coated on the inside with a coating of low-loss conducting material, such as silver, with a thickness of a few skin depths at the atomic resonance frequency. The end walls 23 of the cavity 6 are formed by conductive plates, as of aluminum, aflixed to the ends of the quartz cylinder 22 via the intermediary of flexible longitudinally slotted cylindrical segments 24, as of thin aluminum. The end walls are preferably made of a material having a higher thermal coefficient of expansion than the side Walls 22 of the resonator and cylindrical segments 24 to provide temperature compensation of the resonant frequency of the resonator. Annular recesses 25 are provided between the end wall and the cavity side wall to attenuate undesired TM modes that might couple to or interfere with the desired mode. The cavity end walls 23 each include a centrally disposed threaded insert for trimming the tuning of the end resonator chambers 8.

A small diameter hollow dielectric tube 26, as of quartz, is coaxially disposed of the cylinder 22 and is coated at 27 over a preponderance of its length on the exterior surface with a conductive material, such as silver, to produce an R.F. field free region 28 Within the interior of the tube 26. The conductive coating is terminated :short of the end walls 23 at 29 thereby terminating the center conductor such that the space 8 remaining between points 29 and the end walls 23 is dimensioned of suflicient length (approxi mately k 2 long) to support the TE circular electric mode and permit the RR magnetic fields of this mode to extend into the beam path 4 in this region of the cavity structure 6, thereby defining the pair of axially spaced beam-field interaction regions 8. Regions 8 are coupled together by a resonant section of coaxial transmission line 31 operating in a circular electric mode of the TE configuration.

Wave energy is coupled into the circular electric mode resonator structure 6 via the intermediary of a shallow height section of arcuate rectangular waveguide 32 (see FIG. 3) formed by conductive channel housing member 33 strapped, as by straps not shown, in electrical contact with the outer silvered surface of the quartz cylinder 22. A pair of conductive end walls 34 short the opposite ends of the rectangular waveguide 32. An axially-directed elongated coupling iris 35 is cut through and conductively plated through the wall of the cylinder 22 for coupling to the circular electric mode of the resonator 6 at a central point of symmetry. A similar iris 36 is cut through one of the end walls 34 of the rectangular waveguide 32. A short arcuate section of waveguide 37 interconnect the two irises 35 and 36 for heavily coupling wave energy therebetween.

A pair of inductive vane members 38 produce a strong reflective discontinuity in the waveguide 32 and define a coupling iris 39 therebetween and thus also define the outer terminal boundary of the feed arm portion of the cavity resonator structure 6. A resistor card 41 is disposed across the feed arm portion of the guide 32 from one broad wall to the other to heavily load the composite cavity resonator 6 defined by the feed arm portion and the high Q circular electric mode portions 31 and 8 to lower the composite Q of the entire resonator structure 6 without introducing loss into the high Q portion, thereby rendering the entire cavity relatively insensitive to thermal detuning effects according to the teachings of the afore mentioned patent application Ser. No. 340,767 now US. Patent No. 3,354,307. Wave energy is coupled into the feed arm waveguide 32 via a coaxial line 42 and inductive coupling loop 43. The coaxial line 42 is directed axially of the tube and is connected to a source of microwave power at the frequency of the atomic beam resonance disposed externally of the tube 1.

The cylindrical magnetically-shielded solenoid 7 (see FIG. 2) coaxially surrounds the cavity 6 and includes a cylindrical coil form 44, as of aluminum, grooved on the outer surface and coated with insulating material to form an insulative coating to receive multiple turns of aluminum wire 45 forming the C-field solenoid 7. A cylindrical magnetic shield member 46, as of a material sold by the Allegheny Ludlum Steel Corp. under the trademark Moly Permaloy, coaxially surround the solenoid 7 to shield the interior of the solenoid from extraneously produced magnetic fields including the earths field. A pair of centrally apertured annular magnetically permeable end walls 47 close off the ends of the cylindrical portion 46 of the solenoid shield.

An outer cylindrical magnetically permeable solenoid shield 48 coaxially surrounds the inner shield and likewise includes centrally apertured annular magnetically permeable end closing walls 49.

Referring now to FIGS. 1, 2, 4 and 5, circular electric mode cavity structure 6 and the associated solenoid 7 and magnetic shields are all supported Within the combined cylindrical vacuum envelope 2 and support structure in a self-jigging manner such as to readily achieve and maintain proper transverse alignment of the cavity structure 6 in the following manner: The cylindrical outer magnetic shield 48 is formed by a rolled sheet of metal and includes merely an overlapping slidable abutment of the axial marginal edge portions of the outwardly tensioned cylinder 48. In this manner the cylinder 48 is free to expand out against the inner jigging surface of the inner bore of the cylindrical envelope 2, as of precision bore non-magnetic stainless steel tubing. A pair of annular diaphragms 51, as of stainless steel, jig to the inside surface of the cylindrical envelope 2 via the intermediary of the end portions of the outer cylindrical shield 48 and are afiixed thereto via a plurality of circumferentially spaced sheet metal screws 52 threaded through a plurality of inwardy directed tabs 53. The screws also hold the end Walls 49 of the shield to the side walls of the outer shield 48 via the tabs 53. The self-jigging tube construction forms the subject matter of the aforementioned application, U.S. Ser. No. 385,202, now US. Patent 3,348,040.

The inner magnetic shield 46 is indexed at its ends to the annular diaphragm 51 via the intermediary of a pair of convoluted annular spacers 54, as of stainless steel. The spacers 54 are welded to the first header 51 and bear in longitudinal and transverse engagement against a pair of oppositely convoluted portions 55 of the annular end walls 47 of the inner shield 46.

The cavity resonator structure 6 is indexed to and supported from the ends of the inner magnetic shields 47 via the intermediary of a pair of double-convoluted annular spacers 56, a of aluminum. The annular spacers 56 index to transverse aligning interfaces 57 and 58 on the shield end walls 47 and cavity end walls 23, respectively. The center tube 26 for the cavity is supported from and indexed to the cavity end walls 23 via the intermediary of a pair of annular diaphragms 59. The diaphragms 59 axially receive the tube 26 and capture same therebetween by bearing in longitudinal engagement against a pair of quartz washers 61 carried on the tube 26.

The shielded cavity structure is fixedly secured against axial movement to the envelope 2 substantially only at one end by having the outer magnetic shield 48 fixed in the axial direction against stops 62 spot-welded to the inside wall of the envelope 2 and to the shield 48. A helical spring 63, as of beryllium-copper, is captured at one end against the inside surface of the magnetic shield end wall 47 and bears at its other end against the cavity end wall 23 to spring load the various parts and hold all of the headers and spacer members in firm contact with the supporting structure.

One end of the cylindrical solenoid coil form 44 is fixed to the shield 46 via screws 64 and the other end of the coil form 44 is carried from the cavity end wall 23 via a transverse corrugated header 65, as of aluminum, which provides transverse alignment but which will allow relative axial movement between the cavity 6 and the coil form 44.

The above-described self-jigging tube construction is especially advantageous because the tube is made up of parts having greatly different coefficients of thermal expansion and after assembly the tube is evacuated and baked at 400 C. to fully outgas all parts. During the bakeout cycle the quartz cavity cylindrical :side wall 22 expands radially approximately 0.002 while the coil form 44 expands radially approximately 0.130". Similarly, even greater differential expansions are obtained in the axial direction. Proper transverse alignment to approximately 0.001" is required to be maintained after the bakeout cycle over the length of the tube from source 3 to detector 5. The above described self-jigging construction permits the relatively large differential expansion while maintaining the requisite concentricity of transverse alignment.

A similar self-jigging header and spacer support structure is employed for the other elements of the tube structure (see FIGS. 1, 4, and 5), such as the source 3, state selecting magnets 18 and 19, and detector 5. While longitudinal spacing of the elements is not nearly so critical as transverse alignment, proper longitudinal spacing is advantageously obtained by use of spacer ring members 66 or rods, not shown, stacked inbetween successive selfjigging transverse headers 67. The entire stack of elements is then preferably spring-loaded in compression by a crenelated ring spring 68 disposed inbetween the stack of elements and an end closing wall 69 of the envelope 2 which forces the elements down into the support barrel against a suitable stop such as shoulder 70. Note, that if shoulder 70 is used, then stop 62 is eliminated. The end closing wall 69 includes a lip portion 71 which abuts the envelope 2 at its open end and is joined and sealed thereto as by a weld 72 running around the lip portion 71 at 72.

The end closing wall 69 includes a plurality of hermetically sealed feedthrough insulator assemblies 73 for bringing in and out the electrical connections for the various signals and potentials to various elements within the tube. In addition, the end closing wall includes a pinched-off exhaust tabulation 74 for exhausting the tube during processing. Moreover, the various transverse headers within the stack of assembled elements are perforated at 75 to facilitate exhausting of the tube 1 during processing. The tube is pumped in use by means of a conventional getter i-on pump assembly 76 (not sectioned) disposed within the stack of elements. Aligning pins 77 passing axially through aligned openings in the transverse headers 67 and spacer rings 66 and serve to prevent torsional displacement of the various parts relative to each other.

The atomic resonance tube apparatus, previously described, is not limited to cesium or thallium atoms alone. Certain other isotopes of other metals such as, for example, hydrogen and rubidium may be used. Any electron re-orient-ation transition or resonance in atoms or molecules for which the net atoms or molecules angular momentum, f, is an integer in quantum units of Plancks constant, 11, may be used. In general, it is contemplated any suitable molecular or atomic beam or assemblage having desired resonance characteristics may be used. The terms atom or atomic particle as used herein is defined to mean molecules as well as atoms.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An atomic beam tube apparatus including, means for projecting a beam of atomic particles over an elongated beam path, a first state selector, means disposed downstream of said first state selector for exciting resonance of said beam particles, means to feed microwave energy to said resonance exciting means, a second state selector disposed downstream of said resonance exciting means, means disposed downstream of said second state selector for detecting resonance of said beam particles as excited by said exciting means, said resonance exciting means including a composite cylindrical cavity resonator structure having an inner and an outer coaxially disposed tubular conductor members forming the side walls of said resonator structure, said inner tubular conductor member being shorter in axial extent than the outer tubular conductor member to thereby define a pair of spaced cylindrical cavity portions interconnected by a section of coaxial cavity portion, the fields of the cavity mode supported in said pair of spaced cylindrical cavity portions interacting with the beam of particles to excite resonance thereof whereby undesired phase shifts in the alternating fields taken across the transverse dimension of the beam are reduced to produce enhanced frequency stability of the resonance of the atomic particles of the atomic beam tube apparatus, said pair of coaxially disposed tubular members being formed of a pair of quartz tubes coated with a conductive material to form said conductor members and to support the currents of a TE circular electric resonant mode in said composite cavity structure, said coaxial cavity portion including a pair of conductive end walls centrally apertured for the passage of the beam therethrough, said inner quartz tube being longer in the axial direction than said outer quartz tube and being physically interconnected and supported from the end walls of said composite cavity structure at positions externally of said composite resonator, and said inner quartz tube having its conductive coating removed at the opposite ends thereof within said composite resonator structure to define the pair of cylindrical cavity resonator portions.

2. The apparatus of claim 1 wherein said end walls are formed and arranged with respect to said quartz tubular side wall members so that said end wall tend to move axially toward each other with increase in temperature of said cylindrical cavity resonator structure to reduce resonant frequency shifts.

References Cited UNITED STATES PATENTS 2,698,923 1/1955 Edson 333-83 2,972,115 2/1961 Zacharias et al 3313 3,012,170 12/1961 Heil 333-83 X 3,060,385 10/1962 Lipps et al 331-3 WILLIAM F. LINDQUIST, Primary Examiner.

US. Cl. X.R. 

